Micellar copolymerization of associative polymers: Study of the effect of acrylamide on sodium dodecyl sulfate–poly(propylene oxide) methacrylate mixed micelles

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Micellar copolymerization of associative polymers: Study of the effect of acrylamide on sodium dodecyl sulfate–poly(propylene oxide)

methacrylate mixed micelles

Guillaume Bastiat, Bruno Grassl

, Jeanne François

Laboratoire de Physico-Chimie des Polymeres (LPCP), CNRS/UPPA UMR 5067, Helioparc Pau-Pyrenées, 2 Av. du Président Angot, 64053 Pau cedex 9, France

Received 27 January 2005; accepted 31 March 2005 Available online 31 May 2005

Abstract

Mixed micelles of sodium dodecyl sulfate (SDS) and poly(propylene oxide) methacrylate (PPOMA) have been studied in the presence of acrylamide using conductimetry, fluorescence spectroscopy, and small-angle neutron scattering (SANS) under the following conditions:

(i) the SDS–acrylamide binary system in water; (ii) the SDS–acrylamide–PPOMA ternary system in water. The addition of acrylamide in SDS solutions perturbs the micellization of the surfactant by decreasing the aggregation number of the micelles and increasing their ionization degree. The variations of the various micellar parameters versus the weight ratioR=PPOMA/SDS are different in the presence of acrylamide or in pure water. These differences are much more pronounced for the lower than for the higher PPOMA concentrations. There is competition between acrylamide and PPOMA and at higher PPOMA concentration, acrylamide tends to be released from SDS micelles and is completely replaced by PPOMA.

2005 Elsevier Inc. All rights reserved.

Keywords: Mixed micelles; Sodium dodecylsulfate; Steady-state fluorescence quenching; Small-angle neutron scattering; Acrylamide;

Micellar copolymerization

1. Introduction

Amphiphilic copolymers are extremely interesting be- cause their aqueous solutions exhibit rheological properties that can be exploited in many industrial applications[1–3].

The presence of few hydrophobic lateral or terminal groups in a water-soluble polymer allows the formation in water of reversible associations, which play the role of tempo- rary crosslinks. According to the nature and/or the number of such moieties, aqueous solutions can appear as shear- thinning, shear-thickening, thixotropic, or antithixotropic.

An interesting problem, which has been discussed for a long time by several authors, is related to the possibility of mon-

* Corresponding author.

E-mail address:bruno.grassl@univ-pau.fr(B. Grassl).

itoring these rheological properties through the method of preparation of these polymers.

For water-soluble polymers, specialized preparation tech- niques are required, since hydrophilic and hydrophobic monomers are generally mutually incompatible and not sol- uble in the same solvents. Basically, there are two ways to incorporate hydrophobic moieties onto water-soluble polymer chains: direct co-polymerization of hydrophobic and hydrophilic monomers or postmodification of the par- ent water-soluble polymer. Copolymers based on acrylic acid have been obtained by chemical modification of poly- acrylic acid, as well controlled by the French group of Paris [4,5]. With this method, a statistical distribution of hydrophobes can be expected because of the homogeneous reaction medium. Another advantage of postmodification is that commercially available polymers can be used as starting materials: polyethylene oxide or natural polymers (hydro-

0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved.

doi:10.1016/j.jcis.2005.03.093

Fig. 1. Schema of micellar copolymerization (SDS is the surfactant; acry- lamide and PPOMA are the comonomers) and formula of poly(propylene oxide) methacrylate (PPOMA).

xyethylcellulose)[6–9]. Relatively few studies have focused on chemical modification to prepare polyacrylamide (PAM)- or partially hydrolyzed PAM (HPAM)-based associative polymers (HAPAMs)[10–12]. After attempts using hetero- geneous[13,14], inverse emulsion[15], microemulsion[16], and precipitation methods[17,18], the final commonly ac- cepted method is micellar co-polymerization, in which an appropriate surfactant is used to solubilize the hydropho- bic comonomer, as represented inFig. 1. This method leads to polymers with a blocky distribution of the hydrophobes along the backbone[19–23]. Moreover, it is well established that it is possible to control the rheological properties of the obtained polymers simply by playing on the molar ratio of hydrophobic monomer to surfactant, RM. Experiments show that the higher this ratio, the longer the hydrophobic blocks and the higher the solution viscosity. This behavior can be understood from simple considerations: the number of hydrophobic units per block in the copolymer is assumed to be roughly equal to the initial number of hydrophobic monomers per micelle, N_H, and this is usually calculated from the surfactant–monomer composition (ratio R_M) by assuming that hydrophobic monomers incorporate micelles without changes in the aggregation numberN_agand the crit- ical micellar concentration CMC of the surfactant:

(1) NH= [hydrophobic monomer]

([surfactant] −CMC)/N_ag.

To our best knowledge and despite a well-documented liter- ature on the mixed micelles, these assumptions have never been really checked.

We are involved in the synthesis of new PAM-based as- sociative polymers that incorporate short chains of poly(pro- pylene oxide) (PPO) as hydrophobic stickers. Due to the loss of solubility in water of PPO above the “lower critical solution temperature” (LCST) at about 35^◦C[24], thermo- thickening properties are expected when the aqueous solu- tions are heated above this temperature, according to re- sults already obtained by Hourdet and co-workers[25–28].

Such polymers can be prepared by micellar copolymeriza-

Table 1

Summary of the various parameters of the SDS–PPOMA mixed micelle determined in previous work

R CMC (mol L⁻¹) α N_{ag total} N_{ag SDS} N_{ag PPOMA}

0 0.0084 0.371 58 58 0

0.25 0.0070 0.485 47 39 8

0.5 0.0059 0.612 45 32 13

0.75 0.0055 0.616 39 24 15

1 0.0048 0.696 38 21 17

Note. CMC andαare the critical micellar concentration and the ionization degree;N_{ag total},N_{ag SDS}, andN_{ag PPOMA}, the total, SDS, and PPOMA aggregation numbers [31]. C_SDS=0.05 mol L⁻¹. R=PPOMA/SDS (weight ratio).

tion of acrylamide and poly(propylene oxide) methacrylate (PPOMA) (Fig. 1) in sodium dodecyl sulfate (SDS) solu- tions[29]. The amplitude of the viscosity jump at the LCST of the PPO branch was expected to depend on the length of the PPOMA block in the copolymer and consequently on the weight ratioR=PPOMA/SDS. A systematical study of the SDS–PPOMA micellar system, using many complementary techniques (conductimetry, fluorescence, and light and neu- tron scattering)[29–32], has revealed without any ambiguity that the hydrophobic comonomer (PPOMA) strongly mod- ifies the CMC andN_agof SDS. It clearly appeared that the previous assumption expressed by relation(1)could not be valid in this particular case. It should be interesting to check this conclusion with other hydrophobic comonomers.

On the other hand, it is generally considered that in mi- cellar polymerization with a water-soluble initiator, poly- merization of acrylamide starts in the aqueous phase and when the end of the growing chain encounters a micelle, all the hydrophobic monomers polymerize (Fig. 1). Such a scheme justifies Eq. (1). This is not really convincing because it should be more reasonable to think that only acrylamide can easily polymerize under such conditions, if this monomer had no contact at all with SDS. In the first part of this paper, we report a study of the influence of acrylamide on the micellar properties of pure SDS micelle alone, using different techniques to verify if addition of acry- lamide perturbs or does not perturb the micellar properties of SDS. In our previous works, we have demonstrated that our hydrophobic monomer PPOMA incorporates into SDS micelles, by significantly decreasing the SDS aggregation number,N_{ag SDS} [31,32]. The various parameters of SDS–

PPOMA mixed micelles we obtained in the previous paper are reported in Table 1. A structural model of the mixed micelles, described inFig. 2, has been proposed where the double bond is assumed to lie inside the micelles in the hy- drophobic core, whereas the more hydrophilic PPO short chains are at the SDS micelle surface. It was interesting to know how the presence of acrylamide as comonomer in the reaction medium may change this organization. In the second part of this paper, we will discuss the behavior of the SDS–acrylamide–PPOMA ternary system in water.

The techniques used are conductimetry, fluorescence spec- troscopy, and small-angle neutron scattering (SANS).

Fig. 2. Structure model assumed for the SDS–PPOMA mixed micelle [31,32].

2. Experimental

2.1. Materials

SDS, PPOMA (M_w=434 g mol⁻¹), acrylamide, and do- decylpyridinium chloride of the best reagent grade available were obtained from Aldrich and used without further purifi- cation. Pyrene was recrystallized from methanol. Water was distilled three times over quartz. Totally deuterated SDS, SDSd25, and heavy water, D₂O, were used as received from Eurisotop.

2.2. Conductimetry

The conductivity was measured with a Radiometer Co- penhagen CDM92 conductimeter using a two-pole conduc- tivity cell (constant=1 cm⁻¹). The method was described in the previous paper [31]. A series of solutions of the ternary systems SDS–acrylamide–PPOMA were studied by keeping constant (i) the acrylamide molar concentration of 0.5 mol L⁻¹ and (ii) the weight ratio R =PPOMA/SDS fixed at 0, 0.25, 0.5, 0.75, and 1. At first, concentrated solu- tions of SDS and PPOMA in water containing 0.5 mol L⁻¹ of acrylamide, with a given ratioRwere prepared and small aliquots of these batch solutions were then added in a vol- ume of the acrylamide solution (0.5 mol L⁻¹), contained in a double-walled glass vessel thermostated at 25±0.1^◦C.

Conductivity was measured after each addition.

2.3. Fluorescence spectroscopy

The fluorescence spectra were obtained on a Perkin–

Elmer LS50B spectrofluorimeter. The excitation wavelength was fixed at 335 nm and the band passes were set at 2.5 nm for the excitation and the emission. The cell was ther- mostated at 25±0.1^◦C.

The method of steady-state fluorescence quenching, us- ing pyrene as fluorescence probe and dodecylpyridinium chloride as fluorescence quencher, is described in previous paper[31].

Table 2

Scattering length densities (b) and molecular volumes (V) for several com- pounds used in this work

b(10¹⁰cm⁻²) V (ų)

PPOMA 0.34 720.9

CD₃ 9.78 54.3

CD₂ 7.43 26.9

CH₃ −2.19 54.3

CH₂ −2.05 26.9

SO⁻₄ 1.63 57.9

Na⁺ 9.31 3.9

Acrylamide 1.77 33.7

D2O 6.41 30.2

A small amount of a pyrene solution solubilized in methanol was prepared and after evaporation of the methanol, aqueous solutions of SDS (0.05 mol L⁻¹), acrylamide (0.5 mol L⁻¹), and PPOMA (at various weight ratiosR= PPOMA/SDS=0, 0.25, 0.5, 0.75, and 1) were added. These solutions S₁were stirred for one day at room temperature.

The final pyrene concentration is about 5×10⁻⁶mol L⁻¹. In a part of S₁, a given amount of dodecylpyridinium chlo- ride was added and batch solutions S₂ were obtained. S₂ were diluted by the solutions S1of the same ratioRin such a way that the quencher concentration was ranging between approximately [Mi]/5 and [Mi], [Mi] being the micelle con- centration.

2.4. Small angle neutron scattering (SANS)

SANS experiments were performed on PAXY spectrom- eter at the Léon Brillouin Laboratory (LLB) (CEA, Saclay, France). The measurements were made using a wavelength of 10 Å and a sample-detector distance of 2 m. Scattering range was covered from 0.03 to 0.11 Å⁻¹. The temperature was fixed at 25^◦C. The data were corrected for background scattering and detector response and converted to the scatter- ing cross section orI (q)(in absolute units of cm⁻¹) using standard procedure[34].Table 2provides the values of scat- tering length densities for the compounds used in this work.

For the SANS study, batch solutions of SDS or totally deuterated SDS (SDSd25) and acrylamide were prepared in D2O at molar concentration of 0.05 and 0.5 mol L⁻¹, re- spectively, and PPOMA added such as weight ratio R= PPOMA/SDS (or SDSd25) equal to 0, 0.25, 0.5, 0.75, and 1.

2.5. SANS curves treatment

2.5.1. Evaluation of mixed micelle aggregation numbers from the peak position

In a crystallographic way, we can assume that the peak observed in SANS curves at q =q_max, theq value at the scattering peak, corresponds to the scattering from reticular planes of cubic lattices by assuming that the mixed micelles, which are spherical charged objects, adopt a regular space organization of cubic lattice due to the electrostatic repul- sions. Several types of cubic lattices could be considered,

for example a simple cubic sc, a body-centered cubic bcc, or a face-centered cubic fcc structure, andq_maxcorresponds to scattering from (100), (110), and (111) planes, respectively.

(Generally, in order to distinguish the sc, bcc, and fcc lat- tices from each other, the positions of seven successive peaks must be known.) The lattice constant of the lattice,ai, is re- lated to the distance between the planes,dhkl, through the equations

(2.1) a_sc=d₁₀₀,

(2.2) abcc=2^1/2d100,

(2.3) afcc=3^1/2d100.

Considering that (a_i³) is the volume of the cubic lattice, dhkl=2π/q_max, and that there are 1, 2, and 4 scattering ob- jects per lattice for the sc, bcc, and fcc, respectively, it is pos- sible to write that the micellar concentration [Mi] (mol L⁻¹) is proportional to(q_max)³(q_maxbeing expressed in Å⁻¹), [Mi] =a 10²⁷ (3)

(2π/q_max)³N_A,

N_A being the Avogadro number and the prefactor a de- pending on the nature of the cubic lattices, 1, 2^−1/2, and 4·(3^−3/2)for sc, bcc, and fcc, respectively. So it may be pos- sible to evaluate the aggregation numbersN_agof the mixed micelles (the total aggregation numberN_{ag total}and the num- bers of SDS, N_{ag SDS}, SDSd25, N_{ag SDSd25}, and PPOMA, N_{ag PPOMA}per micelle).

(4.1) N_{ag total}=(C_SDS−CMC)+C_PPOMA

C_mic ,

(4.2) N_{ag SDS}=(C_SDS−CMC)

C_mic ,

(4.3) N_{ag PPOMA}=C_PPOMA

C_mic ,

where CMC is the critical micellar concentration, CPPOMA

andCSDS the PPOMA and SDS concentration. In expres- sions(4.1) and (4.2), when deuterated SDS is considered, C_SDSd25 andN_{ag SDSd25} replaceC_SDS andN_{ag SDS}, respec- tively.

2.5.2. Model of mixed micelle and fit of the SANS curves Using the thermodynamical theories for surfactant mix- tures, we have shown that SDS–PPOMA mixed micelles can be described by a spherical core–shell model[32]. Moreover, we assume that the micelle solutions are monodisperse sys- tems. The SANS scattering intensity can be simply written I (q)=nP(q)S(q), i.e., a product of the form factor,P(q) (in this factor is included the scattered length densities), and the structure factor,S(q), withn, the micelle number den- sity[35–37].

The structure factorS(q)corresponds to an organization of the scattering objects in solution, i.e., to the interaction between the scattering particles. Therefore, it is the same for SDS and SDSd25. As pure SDS micelles, SDS–PPOMA mixed micelles can be considered as charged hard spheres.

The general expression forS(q)is

(5) S(q)=

1+n

∞

0

g(r)sinqr

qr 4π r²dr

=1+ng(q), where n is the number of micelles per volume unit and g(q)the Fourier transform of the pair correlation function g(r). An analytical function forg(q)has been calculated for monodisperse solutions of hard spheres of radiusr within the Percus–Yevick approximation and leads to the following expression for the structure factor,

S(q)= (6)

1+24ΦG(2qr, Φ) 2qr

₋1

.

Here,G(2u, Φ) is a function ofu≡qr andΦ is the vol- ume fraction of the spheres. The interaction between charged hard spheres can be described by the same potential of neu- tral hard sphere of radiusr, complemented by a screening Coulomb interaction potentialV (x)given by

V (x)= C_Bz² (7) (1+κr)²

exp[−κ(x−2r)]

x ,

forx >2r, the distance between the particle centers.C_B= e²/ε, where ε is the solvent permittivity,e is the elemen- tary charge, z is the charge number per particle, and κ is the reverse of Debye–Hückel screening length. The structure factor for solutions of charged hard spheres has been calcu- lated by a mean spherical approximation (MSA) by Hayter and Penfold[38]. Nevertheless, this structure factor is valid only for a sufficiently large volume fraction of scattering objects (>0.2). For smaller volume fraction, Hansen et al.

have improved the method and proposed the rescaled MSA (RMSA), which introduces a rescaled hard core radiusr_res larger than the real radiusr[39].S(q)has no unit.

The form factor P (q) corresponds to the shape of the scattering particles and we have considered spheri- cal core–shell particles, described by Pedersen and various authors [35,37,40–43]. For SDS–acrylamide–PPOMA and SDSd25–acrylamide–PPOMA mixed micelles,P (q)will be described by

(8) P (q)=

(b1−b2)V1f (qr1)+(b2−bS)V2f (qr2)2

with

(9) f (x)=3(sin(x)−xcos(x))

x³ .

The hydrophobic core of radiusr₁ has a scattering length densityb₁ and a volumeV₁. The shell extends between r₁ andr₂and has scattering length densityb₂.V₂is the volume of the whole micelle (core+shell) andb_S is the scattering length density of the solvent. The various scattering length densitiesb₁, b₂, andb_Sare calculated from the composition of the core, the shell, and the solvent (Table 2):

• The core, of radiusr₁, is made up of CH3or CD3 and of part of the 11 CH2 or CD2of the surfactant chains.

n_C, the number of methylene groups in the core, is cal- culated fromNag SDSd25,r1,VCD₃orVCH₃, andVCD₂ or V_CH2, the volumes of CD₃or CH₃and CD₂or CH₂re- spectively, as follows:

(10) n_C=4π r₁³/3−Nag SDSd25VCD₃

N_{ag SDSd25}V_CD2 .

• The shell of internal and external radius r₁ and r₂ is composed of (i) the remaining of the methylene groups (CD₂or CH₂), i.e., a volume of(11−n_C)N_{ag SDSd25}× VCD₂, (ii) the sulfate groups, i.e., a volume ofNag SDSd25

×V_SO−

4, (iii) the sodium ions, i.e., a volume of(1−α)

×N_{ag SDSd25}V_Na+ withαthe ionization degree of the mixed micelle, (iv) the D₂O solvating sulfate groups and sodium ions, i.e., a volume of w_SO−

4V_D2O+(1− α)w_Na+VD₂O with wi the hydratation number of the compounds i, (v) the PPOMA macromonomer, i.e., a volume ofN_{ag PPOMA}V_PPOMA, and (vi) the acrylamide monomer, i.e., a volume ofN_{ag acry}V_acry. In this relation, when hydrogenated SDS is considered,N_{ag SDSd25} and V_CD2 are replaced by N_{ag SDS} and V_CH2, respectively.

w_SO−

4 andw_Na+were taken at 5 and 6 as in Ref.[40].

• The solvent is a D2O solution of acrylamide and SDSd25 (or SDS) at the concentration CMC, and of a part of the sodium ions, at the concentration(CSDSd25− CMC)α+CMC.

P (q)is in absolute units of cm².

The prefactorn(in absolute units of cm⁻³) corresponds to the micelle number density and is described by the fol- lowing relations:

n=(CSDSd25−CMC)NA×10⁻³

N_{ag SDSd25} =CPPOMANA×10⁻³ N_{ag PPOMA}

=(C_SDSd25+C_PPOMA−CMC)N_A×10⁻³ (11) Nag SDSd25+Nag PPOMA

.

In this expression, when hydrogenated SDS is considered, C_SDSd25is replaced byC_SDS.

A Fortran program of determination and parameter fit- ting, based on a traditional optimization method called sim- ulated annealing, was used. Several parameters have been optimized:r₁the radius of the core,r₂the external radius of the core–shell model,N_{ag SDSd25}, N_{ag PPOMA}, and N_{ag acry} the SDS, PPOMA, and acrylamide aggregation numbers, respectively, andα the ionization degree of the mixed mi- celle. We have taken the CMC determined by conductimetry.

N_{ag acry}is a new parameter to optimize in relation to the fit of the SANS curves for the system SDS–PPOMA in pure wa- ter, and it is essentially present in the form factor P(q) (shell and solvent composition).

Fig. 3. Variations of the conductivity as a function of SDS concentration for different weight ratios R=PPOMA/SDS:0 (F), 0.25 (2), 0.5 (Q), 0.75 (E), and 1 (") at a constant acrylamide concentration of 0.5 mol L⁻¹. All the curves start from the origin. They have been shifted for better read- ability.

3. Results

3.1. The SDS–acrylamide binary system in water

For ionic surfactants, the variations of conductivity versus surfactant concentration are characterized, as frequently de- scribed in Refs.[33,44–47], by two straight lines 1 and 2 for surfactant concentrations lower and higher than the CMC, respectively (see Fig. 3). The intersection between lines 1 and 2 gives the CMC and the ratio of the two slopes S₂/S₁ leads to an approximate value of the ionization degree of the micelles, α. We have obtained, for SDS in pure water, CMC=8.4×10⁻³mol L⁻¹andα=0.37 (Table 1 [31]). In the presence of acrylamide (0.5 mol L⁻¹) (Fig. 3), the results are shifted with respect to pure SDS and the following results are obtained: CMC=8.9×10⁻³ mol L⁻¹ and α=0.47.

Even if the differences are not very important, this result re- veals an influence of acrylamide on the micellar behavior of SDS. Acrylamide seems to slightly hinder micellization by modifying the equilibrium of the repulsive and the attractive forces between SDS molecules.

Fig. 4a gives the results of the experiments of steady-state fluorescence quenching in the presence of pyrene as fluores- cence probe and dodecylpyridinium chloride as fluorescence quencher (Q). The plots of ln(I₀/I )(I₀the fluorescence in- tensity of pyrene without quencher and I the fluorescence intensity of pyrene in the presence of quencher) versus the quencher concentration are linear whatever the acrylamide concentration [AM], but the slope of these straight lines de- creases when [AM] increases, which indicates an increase of the number of micelles per volume unit[Mi], which is given by

[Mi] = [Q] (12) ln(I₀/I ).

The SDS aggregation numberN_{ag SDS}is obtained by know- ing CMC determined from conductivity measurements

Fig. 4. (a) Variations of ln(I₀/I )versus quencher concentration for various acrylamide concentrations: 0 (F), 0.05 (1), 0.1 (Q), 0.2 (E), 0.3 (2), and 0.5 mol L⁻¹(P). (b) Variation of the SDS aggregation number versus acry- lamide concentration as determined by steady-state fluorescence quenching.

C_SDS=0.05 mol L⁻¹.

(Eq.(4.2)) withC_SDSthe SDS concentration of 0.05 mol L⁻¹. The values ofN_{ag SDS} are reported versus acrylamide con- centration in Fig. 4b. Addition of acrylamide strongly af- fectsN_{ag SDS}, which decreases from 58 in pure water (Ta- ble 1 [31]) to 20 for [AM]>0.2 mol L⁻¹. To our best knowledge, this surprising result is quite new. There is a lack of information about the variation ofNag SDS of ionic surfactants in the presence of water-soluble monomers.

However, Aguiar et al. have studied the aqueous solutions of tetradecyltrimethylammonium (TTAB) in the presence of formamide and shown thatN_agdecreases from 60 to 23 for 60% of formamide[48]. They attribute this result to the par- ticipation of the cosolvent (formamide) in the solvation layer of the polar heads of the surfactant, which may perturb the electrostatic repulsions between the surfactant molecules. If this explanation was correct also in the case of the SDS–

acrylamide system, one could expect that all the acrylamide molecules would not lie in the intermicellar aqueous phase but would be partially located at the surface of the micelles between the polar groups of the surfactant. In order to ver- ify this hypothesis, we have measured the intensity of the first fluorescence emission peak of pyreneI₁versus [AM] as in our previous paper for PPOMA[31]. The results show a strong decrease of the fluorescence intensity by a factor of 17 when [AM]=0.2 mol L⁻¹, which is consistent with direct contact between pyrene and acrylamide molecules and the location of a part of the acrylamide in the micelles. However, this quenching effect of pyrene fluorescence could perturb the measurements of aggregation numbers by static fluores- cence in the presence of another quencher.

It was necessary to confirm the decrease ofN_{ag SDS} by small-angle neutron scattering.Fig. 5a (curveR=0) shows the small-angle neutron scattering curves for solutions of SDS (0.05 mol L⁻¹) in the presence of 0.5 mol L⁻¹of acry-

Fig. 5. SANS curvesI (q)=f (q)for (a) SDS–PPOMA and (b) SDSd25–

PPOMA mixed micelle in D2O, with acrylamide at 0.5 mol L⁻¹, for various weight ratioR=PPOMA/SDS (or SDSd25).R=0 (F), 0.25 (2), 0.5 (Q), 0.75 (E), and 1 (").C_SDS=C_SDSd25=0.05 mol L⁻¹. (—) Theoretical fits of the SANS curves forR=0 to 1.

lamide in heavy water. The curve exhibits a maximum for a value of the scattering vector, q_max, which is generally considered as inversely proportional to the distance between micelles,d_mic, as we have explained in Section2.

For the binary system SDS–PPOMA in pure heavy wa- ter,qmaxof the SANS curves is 0.05 Å⁻¹. So the presence of acrylamide induces a shift ofqmaxtoward higherqvalues, which means that the distance between micelles becomes smaller. One can deduce that the number of micelles per volume unit increases and that the SDS aggregation number decreases. This behavior is in agreement with the fluores- cence and conductivity measurements.

In the presence of 0.5 mol L⁻¹ of acrylamide, one finds N_{ag SDS}=32, 45, and 42, considering that the micelles are organized as a simple cubic sc, a body-centered cubic bcc, or a face-centered cubic fcc lattice, respectively (Eqs.(2)–

(4)). As for the binary system without acrylamide, values calculated for the bcc and fcc lattices are higher than those determined with the sc lattice, values in quantitative agree- ment with our fluorescence measurements[32]. On the other hand, the relatively high difference with the value ofN_{ag SDS} obtained by fluorescence for this system could be explained by the partial quenching of pyrene by acrylamide (a phenom- enon that may perturb the fluorescence measurements) than by errors in the SANS curves interpretation. More, the fit of the SANS curves, shown later in this paper, gives higher

value for N_{ag SDS}. Nevertheless, the whole set of results clearly indicates a strong influence of acrylamide of the mi- cellization of SDS. We conclude that a part of acrylamide be- longs to the micelles probably by solvating the polar groups of the SDS. The interaction between this monomer and SDS can be explained by the presence in acrylamide of a hy- drophobic double bound. Interestingly enough, among the different water soluble polymers, PAM does not interact with SDS [49], while many works have dealt with the forma- tion of poly(ethylene oxide)/SDS complexes[47,50–52]. In PAM, the hydrophobic double bond has disappeared and the function exposed to SDS molecules is the very polar amide group.

3.2. The SDS–acrylamide–PPOMA ternary system in water The conductivity of the SDS–acrylamide–PPOMA ter- nary system in water was studied by keeping acrylamide concentration constant (0.5 mol L⁻¹) and varying the con- centrations of SDS and PPOMA, for various weight ratios R=PPOMA/SDS. From conductivity curves inFig. 3, the CMC andαvalues were determined, and they are reported inTable 3.

In pure water, it has been already described that addition of PPOMA induces a decrease ofN_{ag SDS}and an increase of the ionization degree of the micelles (Table 1 [31]). This was explained by the incorporation of PPOMA into micelles as schematized inFig. 2. In the presence of acrylamide, the ini- tial values of CMC forR=0 is higher than in pure water but one observes a similar decrease of CMC when R in- creases. This indicates that despite the probable presence of acrylamide molecules at the surface of the micelles, PPOMA enters into the micelles as well. The variation ofαversusR in presence of acrylamide seems also parallel to those with- out acrylamide, indicating in both cases that the charge of the micelles is enhanced whenRincreases, which is consis- tent with the decrease ofN_{ag SDS}.

Micellar solutions of a given SDS and acrylamide con- centrations 0.05 and 0.5 mol L⁻¹, respectively, were in- vestigated by static fluorescence. The linear variation of ln(I₀/I )versus the quencher concentration was verified in all the cases, and allows the determination of [Mi].N_{ag SDS} andN_{ag PPOMA}, the respective aggregation numbers of SDS and PPOMA in the mixed micelle, are determined from

Table 3

CMC andα, the critical micellar concentration and the ionization degree;

andN_{ag total},N_{ag SDS}, andN_{ag PPOMA}, the total, SDS, and PPOMA ag- gregation numbers, in the presence of acrylamide at 0.5 mol L⁻¹ R CMC (mol L⁻¹) α N_{ag total} N_{ag SDS} N_{ag PPOMA}

0 0.0089 0.473 20 20 0

0.25 0.0075 0.561 23 19 4

0.5 0.0070 0.635 30 22 8

0.75 0.0067 0.695 41 26 15

1 0.0062 0.729 44 25 19

Note.C_SDS=0.05 mol L⁻¹.R=PPOMA/SDS (weight ratio).

Eqs. (4.1)–(4.3). Let us note that the solubility limit of PPOMA in water is very low and one can neglect the con- centration of free PPOMA in solution. Moreover, acrylamide molecules probably adsorbed at the micelle surface are not considered. The values of aggregation numbers are reported inTable 3as a function ofR.

A comparison betweenTables 1 and 3indicates that the differences between the values of these aggregation num- bers are more pronounced for the lower concentrations of PPOMA (smaller values ofR). ForR=0.75 and 1, one can consider that there is no more influence of acrylamide, while forR=0.25 and 0.5,N_{ag SDS}andN_{ag PPOMA}are both lower in the presence of acrylamide than in pure water. This behav- ior seems to suggest an exchange between acrylamide and PPOMA molecules in the micelles. In an excess of PPOMA, acrylamide could be completely released from the micelles.

Neutron scattering experiments were performed, as al- ready published for the SDS–PPOMA binary system[32], under two different conditions based on the values of the density of scattering length given inTable 2:

(i) Hydrogenated SDS and PPOMA in a solution of 0.5 mol L⁻¹ of acrylamide in heavy water (both SDS and PPOMA contribute to the scattered intensity).

(ii) Hydrogenated PPOMA and deuterated SDS (SDSd25), in the same solvent (acrylamide in D2O) (the contribu- tion of SDSd25 becomes negligible). This condition is very interesting since it makes it possible to determine how PPOMA is incorporated into micelles. We have been able to demonstrate that PPOMA forms a corona around a core of SDS, as schematized inFig. 2.

Fig. 5reproduces the scattering curves obtained with the ternary system by varying theRvalues under these two dif- ferent conditions. One observes in both cases a shift ofq_max toward higherqvalues whenRincreases and it is important to observe that theq_maxvalues are the same under both study conditions. One can consider again the simple arguments for calculating [Mi] (see Eqs.(2)–(4)).Fig. 6gathers the values ofN_{ag SDS}andN_{ag PPOMA}, obtained either from steady-state fluorescence quenching or from SANS. They can be com- pared with those previously determined in pure water.

It is interesting to note the good agreement between the results deduced from fluorescence and SANS, in the absence of acrylamide, whatever the ratio R, considering the sim- ple cubic sc lattice[32]. In the presence of acrylamide, for the higher values ofR, 0.75 and 1, there is also good con- sistency between the results obtained by the two methods (fluorescence and calculation with sc lattice). The calcula- tion of aggregation numbers considering a bcc or a fcc lattice does not lead to suitable values (as for the system without acrylamide). The loss of influence of this cosolvent on the PPOMA–SDS mixed micelles when PPOMA is in excess is well confirmed. However, significant differences in the val- ues of N_{ag SDS} are observed for the smaller R ratios. One can conclude that, even if the agreement between the differ-

Fig. 6. Variation of SDS aggregation numberN_{ag SDS}(fully symbol) and of the PPOMA aggregation numberN_{ag PPOMA}(empty symbol) as a func- tion of the weight ratioR=PPOMA/SDS. The values was determined by fluorescence (F,E) and calculated considering a simple cubic sc (Q,P), a body-centered cubic bcc (",!) and a face-centered cubic fcc (2,1) lattice.

ent sets of values is less good when the effect of acrylamide is preponderant, this cosolvent strongly influences the micel- lization of SDS for lower concentrations of PPOMA.

4. Discussion

Experimental results presented above allow us to con- clude that

(i) the addition of acrylamide in SDS solutions perturbs the micellization of the surfactant, by decreasing the aggre- gation number of the micelles and increasing their ion- ization degree. One can assume, as for similar systems studied by other authors, that acrylamide participates in the solvation of polar heads of SDS and provokes the increase of specific area per polar head at the micelle surface.

(ii) The variations of the various micellar parameters ver- sus the weight ratioR=PPOMA/SDS are different in the presence of acrylamide or in pure water. These dif- ferences are much more pronounced for the lower than for the higher PPOMA concentrations. There is proba- bly competition between acrylamide and PPOMA and at higher PPOMA concentration, acrylamide tends to be released from SDS micelles and seems to be completely replaced by PPOMA.

Three attempts are presented here to confirm these hy- pothesis (i) through a fluorescence quenching experiment, (ii) through the theoretical fits of the SANS curves (Fig. 5), and (iii) through a simple geometrical model.

4.1. Fluorescence quenching experiment

In a fluorescence quenching experiment, the decrease of fluorescence intensity of the probe is related to the forma- tion of a complex between this probe and the fluorescence

Fig. 7. (a) Variation of the fluorescence intensity of the first peak for the pyrene spectrum versus the acrylamide concentration (F), and (b) from Cacry=0.5 mol L⁻¹, versus the PPOMA concentration (E). The two ab- scissas are not on the same scale for a better visualization. (c) The curve cor- responding to the variation of the fluorescence intensity versus the PPOMA concentration (1) without acrylamide has been added for comparison[31].

quencher, the complex not emitting fluorescence[53]. The quencher that we use being dodecylpyridinium chloride, the dodecyl chain is incorporated inside the micelle in the same way as the dodecyl chain of SDS; the pyridinium group forms a complex with pyrene near the surface of the micelle.

Various authors specify that pyrene is located in the neigh- borhood of the surface[53–55]. It is to be noticed that all the molecules having a double bound behave like quenchers for fluorescence probes.Fig. 7presents the variation of the fluorescence intensity of the first peak of pyrene spectrum versus the acrylamide and PPOMA concentrations (or the weight ratioR=PPOMA/SDS).

As we have said in the part relating to the SDS–acryl- amide binary system, pyrene is quenched by acrylamide:

there is a decrease of the fluorescence intensity (curve a). For concentrations higher than 0.2 mol L⁻¹, addition of acry- lamide does not modify the fluorescent intensity of pyrene:

pyrene–acrylamide complexes are not formed any more.

From the acrylamide concentration of 0.5 mol L⁻¹ (con- centration used in this work), we have added PPOMA, and we observed, as shown in curveb, an increase of the flu- orescence intensity and, for a weight ratio R=PPOMA/

SDS=1, the fluorescence intensity of pyrene has practically the same value as that obtained without acrylamide[31], i.e., only with PPOMA (curve c). This increase of fluorescence intensity with the addition of PPOMA well shows that the acrylamide is replaced by PPOMA when the weight ratioR increases.

4.2. Fits of the SANS curves

We have fitted the SANS curves (Fig. 5) with a core–

shell model for the form factor P (q) and a structure fac- tor S(q) of hard sphere with electrostatic repulsion for the two systems: SDS–acrylamide–PPOMA and SDSd25–

Table 4

Values of the optimized parameters

Weight ratioR=PPOMA/SDS

0 0.25 0.5 0.75 1

SDS–acrylamide–PPOMA micellar system

r₁(Å) 15.4 13.3 12.5 11.8 11.1

r2(Å) 19.9 19.2 18.3 17.7 17.7

r=r₂−r₁(Å) 4.5 5.9 5.8 5.9 6.6

N_{ag SDS} 44 32 26 22 20

N_{ag POPMA} 0 6 11 14 19

Nag acry 19 11 7 4 1

α 0.274 0.395 0.509 0.615 0.682

n_C 11.0 9.4 9.7 9.7 8.5

SDSd25–acrylamide–PPOMA micellar system

r1(Å) 11.9 11.2 10.3 10.6

r₂(Å) 16.3 16.1 15.6 15.0

r=r₂−r₁(Å) 4.4 4.9 5.3 4.4

N_{ag SDSd25} 27 26 22 21

N_{ag POPMA} 6 11 14 18

Nag acry 12 11 5 2

α 0.266 0.415 0.543 0.612

n_C 7.8 6.5 5.7 6.7

Note. The core radius isr₁, the external radius of the shell isr₂, the SDS (or SDSd25), PPOMA, and acrylamide aggregation numbers areN_{ag SDS} (orN_{ag SDSd25}),N_{ag PPOMA}, andNag acry, respectively, the ionization de- gree isα, and the number of methylene groups of SDS (or SDSd25) in the core isnC, for SDS–acrylamide–PPOMA and SDSd25–acrylamide–

PPOMA mixed micelles for various weight ratioR=PPOMA/SDS (or SDSd25).C_SDS=C_SDSd25=0.05 mol L⁻¹.

acrylamide–PPOMA in D₂O. The only difference from our previous study (SDS–PPOMA mixed micelle without acry- lamide[32]) is that we have considered that acrylamide is present in the shell of our system. So, fitting the SANS curves, one can evaluate this number of acrylamide. The theoretical fits are reported inFig. 5. We observe that the theoretical fits seem to be adequate and the values of the op- timized parameters are reported inTable 4.

For the binary system SDS–acrylamide, the aggregation number of SDS is 44, a value higher than those determined by steady-state fluorescence quenching. Acrylamide well disturbs the fluorescence quenching effect of dodecylpyri- dinium chloride. Nineteen acrylamide molecules are found to be located in the shell of the micelles.

When the ratioR=PPOMA/SDS increases from 0.25 to 1, we observe a decrease ofN_{ag SDS}from 27–32 to about 20, and an increase ofN_{ag PPOMA}from 6 to 18–19. WhateverR, the values ofN_{ag PPOMA}determined for the two independent systems are very close. Besides, whenRincreases from 0 to 1,Nag acry decreases from 19 to 1–2. This is in agreement with the previous fluorescence experiment, which suggested that PPOMA replaces acrylamide.

It is very difficult, as in our previous study[32], to con- clude on the ionization coefficientα. Nevertheless, we ob- serve thatα increases when the ratioR increases and this parameter follows the same qualitative variation as deter- mined by conductivity.

About the core–shell dimension, whenRincreases from 0 to 1,r₂ decreases from 19.9 to 15–17.7 Å. The radius of

the whole micelle decreases by about 2–5 Å. The same vari- ation is obtained for the core radiusr₁, which passes from 15.4 to 10.6–11.1 Å. In fact, the depth of the shell r re- mains practically constant, and this is more marked for the SDSd25–acrylamide–PPOMA system: about 4.5 Å. We can also remark thatn_C, the number of CH₂(or CD₂) groups of SDS chains composing the core, decreases from 8–9 to 7–8 whenRincreases from 0.25 to 1. For SDS micelle (R=0), this value is 11. At first, acrylamide does not go very deeply inside the SDS micelle. When PPOMA replaces acrylamide, this one goes slightly more deeply inside the mixed micelle.

We can also remark that the 19 acrylamide monomers ini- tially present in the SDS micelle (whenC_acry=0.5 mol L⁻¹) are totally replaced by 19 PPOMA macromonomers.

4.3. Geometrical model

For all that follows, we have used the aggregation num- bers obtained by the fits of the SANS curves. We can con- sider that the acrylamide is a cosolvent for H₂O, as Aguiair et al. have made for the study of tetradecyltrimethylammo- nium bromide (TTAB) in a formamide–water mixture[48].

For pure SDS micelles, we can consider that our micellar system is spherical (model used for SANS curve fits), and the two opposite forces that control the assembly (attraction of the hydrophobic parts of surfactant and repulsion of its ionic heads) act mainly at the hydrophobic core–water in- terface and tend to decrease or increase the optimal surface, denoteda₀, by molecules at the interface. The micelle geom- etry is defined by the optimal surfacea0of the sulfate groups and the volume v of the hydrocarbon chain C12, located only in the micelle core, assumed to be fluid and incom- pressible. Volume v is a semiempirical parameter and for a hydrocarbon chain saturated withncarbons, it was shown by Israelachvili that v≈(27.4+26.9n)×10⁻³ nm³[56].

For a spherical micelle, withr_chthe radius of the hydropho- bic core,a₀the optimal surface per polar head, andN_{ag SDS} the aggregation number, we have

(13.1) r_ch= ³

3vN_{ag SDS}

4π ,

(13.2) a₀= 4π r_ch²

N_{ag SDS}.

From the data of the binary system,C_acry=0.5 mol L⁻¹, we obtainr_ch=1.54 nm anda₀=0.681 nm²forN_{ag SDS}=44, andv=0.35 nm³.

Besides, as we have done for the micellar system in pure water[31], we can consider that the methacrylate function of PPOMA participates in the total hydrophobic core surface S_mic, which is given by the relation

S_mic=N_{ag SDS}a₀+N_{ag PPOMA}a_PPOMA, (14)

where aPPOMA is the surface of the methacrylate func- tion (considered as a plane in the first approximation, a_PPOMA = 0.283 nm²). From r_ch values, depending on

Table 5

Radiusr_chof the hydrophobic core and optimal surfacea₀of the sulfate group of SDS versus the ratioR=PPOMA/SDS.C_SDS=0.05 mol L⁻¹ andCacry=0.5 mol L⁻¹

Weight ratio PPOMA/SDS

N_{ag SDS}/N_{ag PPOMA} r_ch(nm) a₀(nm²)

0 44/0 1.54 0.681

0.25 27–32/6 1.31–1.39 0.704–0.738

0.5 26/11 1.30 0.691

0.75 22/14 1.23 0.677

1 20–21/18–19 1.19–1.21 0.616–0.628

N_{ag SDS}, it is possible to calculate a₀ for the various ratios R=PPOMA/SDS, defined by the relation

a₀=S_micelle−N_{ag PPOMA}a_PPOMA N_{ag SDS}

=4π r_ch² −N_{ag PPOMA}a_PPOMA (15) Nag SDS

,

r_ch being calculated from Eq. (13.1). All these values are reported inTable 5.

First, we can remark that when we increase the ratio R from 0.25 to 1, the optimal surfacea₀of the SDS decreases from about 0.72 to 0.62 nm².N_{ag SDS}=44 for a pure SDS micelle withC_acry=0.5 mol L⁻¹, determined by the fit of the SANS curves, seems to be a little high: thea₀value for R=0 is lower than this one forR=0.25. It is interesting to note that for the ratioR=1, thea₀value is the same as that found for our SDS–PPOMA micellar system in pure water, i.e., 0.621 nm²[31].

As Aguiar et al. have shown for formamide interaction with TTAB micelles[48], we can conclude that the decrease in the surface area per headgroup of the surfactant (when R increases) is totally in agreement with the formation of micelles with a higher degree of solvation, probably due to the participation of the cosolvent in the solvation layer of the micelle headgroup. So, for the low ratioR, acrylamide is present at the hydrophilic–hydrophobic interface in the mixed micelle. The fact that, for the higher ratiosR, thea₀ value is the same as in pure water means that there is no acrylamide at the hydrophilic–hydrophobic interface, a con- clusion in agreement with the result of SANS and quenching experiments.

4.4. About the micellar copolymerization of acrylamide and PPOMA

Finally, all the experimental data are consistent with the model presented in Fig. 8a, where acrylamide lies at the hydrophilic–hydrophobic interface. When hydrophobic ad- ditive (PPOMA) is absent and in the presence of 0.5 mol L⁻¹ of acrylamide, the aggregation number of SDS is ranging from 20 to 44 according to the experimental method of inves- tigation (fluorescence or SANS, respectively). In this range, the number of acrylamides per SDS polar head varies be- tween 0.5 and 1. Nevertheless, when PPOMA is added, there

Fig. 8. (a) Structure model assumed for the SDS–acrylamide–PPOMA mixed micelle. (b) Micellar copolymerization. Case 1, the polyacrylamide chain in growth does not polymerize PPOMA and N_H =0. Case 2, the polyacrylamide chain in growth polymerizes a part of PPOMA and 0< N_H < NH. Case 3, the polyacrylamide chain in growth polymerizes all PPOMA andN_H =NH.

is a competition with acrylamide, which is progressively re- leased from the surface of the micelle, and in a large enough excess of PPOMA, the micelle again becomes close to the model represented inFig. 2.

It is clear that this finding has important consequences for the micellar synthesis of acrylamide-based copolymers.

Acrylamide modifies the number of SDS per micelle, which is a reason to prove that the assumptions implied in Eq.(1), generally used to calculate the hydrophobic block length, are wrong. Besides, the fact that acrylamide is located in the solvation layer of the micelle is important because this monomer allows the chain in growth to enter and leave the micelle. Indeed, normally, in the case of SDS surfactant, it should be impossible for a radical chain in growth to enter into an object with a negatively charged shell on the surface.

We have here a better understanding of the micellar copoly- merization.

Moreover, at least in the case of our PPOMA macromon- omer, there will be variations on the copolymer structure (illustrated by Fig. 8b), not to mention here the problem of variation of copolymer molar weight and polydispersity

and the problem linked to difference on reactivity ratio val- ues between acrylamide and PPOMA. The two polymeriz- able functions of PPOMA and acrylamide are located at the hydrophilic–hydrophobic interface of the SDS micelle.N_H being the number of PPOMA macromonomer per micelle and N_H the number of PPOMA per bloc in the copoly- mer, three cases can be considered: (i) the polyacrylamide chain in growth can enter into a mixed micelle and go out of it without polymerizing PPOMA, soN_H =0 (case 1 in Fig. 8b), (ii) the polyacrylamide chain in growth can en- ter into a mixed micelle, polymerize all the PPOMA, and leave it so thatN_H =NH (case 3 in Fig. 8b), and (iii) the polyacrylamide chain in growth can enter in a mixed mi- celle, polymerize a part of PPOMA, and go out of it so that 0< N_H < N_H(case 2 inFig. 8b). In all the cases, we can re- mark that the hydrophobic blocks in the copolymer not only will be constituted by the hydrophobic units but will include acrylamide units as well, and owing to the monomer compo- sition (linked to the ratioR=PPOMA/SDS), the number of acrylamide units can vary.

5. Conclusion

We have studied in this paper the interaction of acry- lamide with sodium dodecylsulfate (SDS)–poly(propylene oxide) methacrylate (PPOMA) mixed micelles using con- ductimetry, fluorescence spectroscopy, and small-angle neu- tron scattering. The most interesting result of this paper is the demonstration of interactions between acrylamide and sodium dodecyl sulfate, which have never been described.

All the experimental data are consistent with the model pre- sented inFig. 8a. When acrylamide is added, we observe a decrease of the SDS aggregation number, ranging between 20 and 44 according to the experimental method of inves- tigation for an acrylamide concentration of 0.5 mol L⁻¹. When PPOMA is added, the acrylamide monomer is pro- gressively released from the surface of the micelle, and for a weight ratio PPOMA/SDS=1, the structure of the micelle again becomes close to the model represented inFig. 2. The acrylamide is practically totally released.

This finding has important consequences for the micellar copolymerization of acrylamide and PPOMA. From all this, it can be concluded that attempts to establish clear relations between the experimental synthesis conditions (composition of the reactional medium) and the associative properties of the aqueous solutions of the copolymers will not be very easy in our particular case: the SDS–PPOMA micellar sys- tem. More, for other monomer–surfactant systems, it should be necessary to study the micellar parameters of the reac- tional medium before studying the copolymer properties.

Acknowledgments

We gratefully acknowledge William Binana-Limbele for the fruitful discussions about fluorimetry experiments. We

also thank Alain Lapp for the SANS measurements at the Laboratoire Léon Brillouin (CEA, Saclay), and Oleg Borisov and Frederic Dinand for their help about the data treatment of SANS curves.

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